1.Field of the Invention
The present invention relates to a synchronized receiver for a spread spectrum communications system, and more particularly to a system for synchronizing the pseudo noise code transmissions between receiver and transmitter in a spread spectrum system.
2. Description of the Related Art
Spread Spectrum wireless systems transmit signals over a bandwidth that is much greater than required for standard narrow band transmission. The purpose of this approach is to minimize noise interference, reduce the likelihood of narrow band interference and multi-path effects, and provide security against unauthorized eavesdroppers. In the past, the use of spread spectrum techniques were limited mostly to military applications. However, spread spectrum systems are now becoming dominant in the areas of satellite communications, point-to-point connections, wireless local loop communications, wireless local area networks and digital cellular communications.
The two most common types of spectrum spreading are frequency hopping and direct sequence techniques. Both approaches utilize a pseudo-random noise (PN) code generator in the transmitter to spread the RF carrier signal over a wide bandwidth. A code generator in the receiver having the identical PN code "despreads" the signal to recover the data stream. These techniques offer the advantage of having multiple simultaneous communication links on the same frequencies by assigning them different spreading codes. Each transmitted signal looks like noise but can be decoded by a receiver with the same PN code to regenerate the original data stream.
The frequency hopping (FH) spread spectrum process causes the carrier signal to "hop" from one frequency to another using a hop table. The receiver hops over the same hop table as the transmitter and then demodulates the carrier to receive the data.
The direct sequence (DS) spread spectrum technique, which may be used for code division multiple access (CDMA), encodes the binary or analog data itself into a high-speed PN code. This is done by mixing the data with a much faster sequence of PN code bits, wherein each bit of data is converted into a number of sub-bits, called "chips." This coded data signal is then mixed with the RF carrier signal to effectively spread the main lobe of the carrier over a bandwidth spectrum equal to twice the clock rate of the chipping code. Direct Sequence transmission changes the nature of a transmitter station from predominately analog processing to mostly digital. Analog devices are still generally used for modulating and demodulating carrier signals with the data stream. However, the pseudo-random techniques which spread the signal frequency by code-modulating the transmitted signal are applied and removed in the digital domain. Using DS, the conventional communication waveform is multiplied by a PN binary sequence in the transmitter. Hence, the amount that the signal is spread is determined by the chipping rate. In the receiver, the same PN code is mixed with the received signal to despread the information and recover the original modulated narrow band signal.
In all spread spectrum systems it is essential that the receiver apply the same PN code synchronized in the same phase as the PN code emanating from the transmitter, in order to decipher the data stream. The synchronization occurs in two steps: acquisition and tracking. In acquisition, a coarse estimate of the timing offset is produced by a correlation device. Then this estimate is refined to the point where reliable data demodulation can occur by a tracking device, such as a phase lock loop (PLL). The PLL holds the receiver PN sequence in the proper phase with the transmitter PN sequence to collapse the spread signal back to the original narrow bandwidth centered at the modulated carrier frequency.
Various synchronizing techniques have been developed for synchronizing a code generator in a receiver with the code sent by a corresponding transmitter. In one synchronizing process called "carrier lock tracking," the receiver has a sliding correlator which phase aligns its locally generated code sequence with the transmitted code sequence to find a "carrier lock." A carrier detection/frequency division process is then used to maintain the lock. One disadvantage to this approach is that the clock frequency for the receiver code generator is required be a sub-multiple of the carrier frequency, resulting in undesirable design constraints.
In U.S. Pat. No. 5,101,417 (Richley et al.) a variation of carrier lock tracking is disclosed in which a local clocking generator provides a nominal frequency which is offset from the transmitted code frequency. Therefore, the code sequence of the receiver "slides" into phase correlation with the code sequence of the transmitted signal. A phase controller regulates the rate at which its resident code generator is clocked, once phase correlation has been achieved. After initial correlation, a lock detection circuit determines when the coded sequence of the receiver begins to slide out of phase with the transmitted code and generates an error signal which triggers the phase shift circuit to increase or decrease the clock frequency until the code sequences are aligned.
This approach has the disadvantage that the locally generated code sequence tends to slide out of phase alignment with the transmitted code sequence and must be almost continually re-aligned by a triggering circuit which phase shifts the clock pulses of the locally generated code sequence. Thus the frequency of the local code sequence is essentially continuously variable, similar to a VCO in a traditional phase-locked loop system. This process requires a constant error signal to maintain alignment of the receiver and transmitter code generators.
Moreover, prior art receivers for spread spectrum systems, such as the Richley system, generate a digital feedback error signal after the signal has been demodulated, resulting in some delay in feedback and a certain amount of power loss.
Some prior art communication systems provided an analog signal from the receiver modulator to an analog peak detector that sent an analog error signal to be processed by a locking circuit. However, the analog signal from the detector tended to be unstable and required substantial temperature stability and very limited component variables.
Accordingly, it is apparent that there is a need for a method and system for synchronizing spread spectrum communications systems which processes an analog peak signal generated directly by a spread spectrum receiver without encountering the instability of prior art systems. Moreover, there is a need for a method and system which will achieve phase lock of the code sequences without requiring that the receiver clock frequency be a sub-multiple of the carrier frequency and without needing a constant error signal to maintain alignment.